The Leopard is very definitely NOT based on a Leopard 1 chassis. People are always fooled by the look of the turret which is similar to Leopard 1A3/1A4. Early Leopard 2 prototypes did follow the same basic design principles as Leopard 1, but everything is bigger, including the turret which was also more angled, especially at the front. The Leopard 2 give-aways are the full-width twin cooling grilles in the rear deck and the full-width grille across he back.
It wasn't until later that Leopard 2 changed from using sloped armour to the vertical armour we are familiar with.
There is a similar Leo 2 prototype at the Saumur Museum and another at Trier.